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Flow calculations...

  1. Dec 15, 2016 #1
    So, I am new to here and I am currently a technician that does vacuum systems for the semiconductor industry (mainly Electrical). I came across something today that has me quite perplexed.

    So We have this system that flows gas through 1/4" stainless tubing to a pump to dilute gasses and chemicals inside. For testing purposes we run compressed air just to cool the seals. From my understanding the fabs run N2 through them for purge.

    My issue is that some systems we get good flow rate through at 6-9 SLM. others will flow lower. Our alarm for the flow is 5 SLM but our flow averages between 5.4 and 6.4 SLM.

    I think our specs are wrong as we have to make them because there is no manual for these pumps that is in English. Another tech says it's the solenoid that restricts the flow through the line set. When you look up the spec for flow rate on the solenoid it gives you 0.27CV. If I convert this to scfm at 90 and 100 psi I get a minimum flow rate of 14.0625 SCFM and a max of 15.2542 SCFM. We do not use SCFM so when I convert that to SLPM I get minimum of 398.21 SLPM and max of 431.95 SLPM which indicates that this specific solenoid does not restric our max flow which in the system is 60 SLPM.

    Does this sound legitimate?
    I have read that there is no pressure and flow relationship but I don't quite understand why.. I would think that more pressure would increase flow in a closed system like this because higher pressure=higher pressure differential between input and output.

    Basically put I am trying to confirm that this solenoid has no way of restricting this flow rate lower than the 60 SLM. How would I legitimately go about figuring out the flow rate of this Solenoid?
  2. jcsd
  3. Dec 16, 2016 #2


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    I did a quick Cv calculation for an inlet pressure of 100 psi air which confirms that your solenoid valve should easily flow what you require with a less than 1 psi pressure drop, based upon that, if the solenoid is functioning correctly, it not having any effect on your flow rate.

    The best test to confirm if that is true is to measure the flowing system's pressures immediately upstream and downstream of the solenoid valve to see if there is any measurable pressure drop.
  4. Dec 16, 2016 #3
    Ok, Sweet. I did not think it would affect it in any way. Was my CV calculation correct?

    This is what I personally did (I do not know exactly what CV is, well I think it is constant velocity however, I am not familiar with those measurements):

    I calculated a min and max because we range between 90 psi and 100psi

    For 90 psi: I did 0.27/0.0192=14.0625 SCFM then converted that to SLPM (which I am familiar with) for 398.21 SLPM

    For 100 psi: 0.27/0.0177= 15.2542 SCFM then converted to SLPM for a max flow rate of 431.95 SLPM

    The conversion from SCFM to SLPM I did not know so I used a calculation tool to get to that point. Would I be correct in my assumption and if not how exactly would I do the math to figure it out exactly?

    Also is that what Cv is? if not, What is CV? Sorry for the million questions in my reply. I have never taken mechanical engineering classes or anything of the sort. I'm not a huge mechanical guy personally. I work more on the electrical side with circuit boards and the like.
  5. Dec 17, 2016 #4


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    I am not familiar with the method you used for your calculation so I am attaching a copy of the standard I used and I am including a screen print of my calculation using this method for the 100 psig case that gives a different flow rate than what you calculated. There are two calculations, one that provides the maximum flow for the .26 Cv regardless of how low the downstream pressure is; and, a second that shows the minimum flow that would occur if the solenoid valve downstream backpressure is as high as 99 psi. For your case, for your case the actual flow will fall somewhere between these two points depending upon the downstream tubing backpressure. The alternate calculation method shown on the attached will also allow you to do a reverse calculation using your desired flow rate to calculate the Cv values needed to provide that rate.

    upload_2016-12-17_0-0-46.png upload_2016-12-17_0-0-46.png

    Attached Files:

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